The honda engine which the brawn used was not that good either. It was down on power and drivability iirc. And when brawn won the championship they used mercedes engines. Not honda engines. Also if you look at f1 for the past 20 years the mercedes engine has been consistantly the best one or one of the best ones. At some point the bmw might have made more power or the renault was more fuel efficient but the mercedes has been the engine to have for a very long time. In the meanwhile honda hasn't really created anything extraordinary in the same timeframe.
I can't really blame ron and mclaren for going with honda though. With the new hybrid engines the only way to fight for championships is to either be an engine manufacturer or have engine manufacturer creating engines just for you. Mclaren would never be able to challenge mercedes factory team with mercedes engine because mercedes would never sell/send equal engines to anyone who could beat them on track with them.
Then when you add the huge cost of the engines you are really looking at bad odds. At least now mclaren doesn't need to pay all of this. Honda is paying a lot of money too. But my point is jumping ships to mercedes engines is not a magic bullet. At best the mercedes would sell something like year old engines at full price for mclaren. Your chassis needs to be magical if you want to beat the possibly best chassis of mercedes with worse engine! (although this year ferrari might have the best chassis).
Another thing is the money. The amount of money honda is pouring into mclaren would simply be taken out of mclaren's own pockets. And we are talking high 8 figure sums, possibly even low 9 figures (100,000,000$+).
